1. K Müller and D Wintgen,J. Phys. (Special issue on atoms in external fields)B27, 2693 (1994)
2. M Courtney, H Jiao, N Spellmeyer, D Kleppner, J Gao and J B Delos,Phys. Rev. Lett. 74, 1538 (1995)
3. A Holle, J Main, G Wiebusch, H Rottke and K H Welge,Phys. Rev. Lett. 61, 161 (1988)
4. J-P Connerade,J. Phys. B30, L31 (1997)
5. J-P Connerade, G Droungas, R Elliot, X He, N Karapanagioti, M A Farooq, H Ma, J P Marangos and M Nawaz,J. Phys. B27, 2753 (1994)